The invention relates to a method for operating a hybrid drive for a vehicle and to a corresponding hybrid drive.
A hybrid drive, which is typically used in a motor vehicle, is characterized in that at least two different energy converters and two different energy storage devices are used for traction purposes, i.e. for driving the motor vehicle. Advantages of the hybrid drive result from lowering the energy consumption and reducing exhaust gas and noise emissions.
Motor vehicles, in which a hybrid drive is used, are also denoted as hybrid vehicles. In a type of hybrid vehicle which is already frequently in use, an electrical drive is available in addition to the internal combustion engine to provide the driving force or rather forward propulsion of said vehicle. In so doing, there are various embodiments in which the electrical energy is used for the propulsion of the vehicle.
In the American patent publication US 2002/174796, a drive for a motor vehicle is described, which comprises an energy storage and regeneration system. This system makes energy available for use when required. Braking energy is in turn stored in the system. The system can comprise a flywheel, a battery, a capacitor or a combination of these energy storage devices.
An embodiment of a hybrid vehicle, which is denoted as an axle hybrid, provides that the direct drive of one vehicle axle takes place with an electric machine, that is to say with an electric motor, whereas the second axle is driven with an internal combustion engine. The electric axle can be used to drive the vehicle as well as to charge the battery.
In an operating mode of the axle hybrid vehicle, provision is made for a purely electric driving operation, in which the internal combustion engine is at rest and the forward propulsion results only with the electric motor. In order to be able to use said internal combustion engine for the forward propulsion of the vehicle, said engine must initially be started. This can result on the one hand using a conventional starter, respectively starter motor, which is subsequently denoted in the present application as a starter motor start, or on the other hand while driving by means of tow-starting the internal combustion engine by the electric motor, wherein the kinetic energy of the vehicle is used. The latter is referred to as a road start in the present application. A set of conditions exists for deciding whether to start said internal combustion engine. Said conditions comprise, e.g., the state of charge of the battery (SOC: state of charge), the vehicle speed, the torque desired by the driver, wheel speeds, yaw rate and the steering wheel angle.
Because the number of internal combustion engine starts, which can be carried out via a starter, are limited by the service life of the starter motor, it is recommended to use the road start as frequently as possible to start the internal combustion engine.